1. Field of the Invention
The present invention relates to an optical information record medium that is recordable and has a multi-layered structure having two or more layers such as a two-layered version of CD-R/RW, a two-layered version of DVD±R/RW, or a future multi-layered record medium, and also relates to a method of defining the characteristics of such medium, a method of examining the characteristics of the medium, a method of detecting a signal, a circuit for detecting a signal, and an optical information recording/reproducing apparatus.
2. Description of the Related Art
It can be said that in recent years, CD-R/RW and DVD±R/RW have been established as an external storage device for PCs. These CD-R/RW and DVD±R/R are expected to provide further increased capacity, and study has been made with respect to the two-layered structure and also a structure for future with further increased numbers of layers.
In such multi-layered media, the distance between layers is set such that light reflected by a record layer different from the record layer being accessed does not interfere with the light reflected from the record layer being accessed. At a point far away from the focus point, light reflected from the medium disperses without being condensed by a lens, resulting in a level that is ignorable as signal magnitude. Optical limitations are imposed, however, such that all the multi-layers need to be disposed within a focus tolerable range (focus depth) that allows various satisfactory characteristics to be obtained. Because of this, the inter-layer distance cannot be widened as much as desired. As a result, interference with the light reflected from other layers occurs to no small extent.
An example of a multi-layered structure that has been put to practical use includes a two-layered version of DVD-ROM. This structure has two layers stamped at the factory and designed for Read-Only, so that a large margin is provided against various fluctuation. For the tracking purpose, the DPD (differential phase detection) method that uses a phase difference derived from the edges of a reproduced signal is employed, which provides a strong tolerance against changes in the light intensity (signal magnitude) caused by the interference with light reflected from other layers. Because of the Read-Only configuration, there is no need to detect a track and/or a wobble resulting from the wobbling of the track (groove). Trouble due to the interference with light reflected from other layers is thus rare.
A medium having a plurality of recordable layers is disclosed in Patent Document 1, 2, and so on, for example. These inventions improve the feasibility of multi-layered recording by specifying the record film characteristics such as record film thickness and record film material of the media designed for multi-layered recording. Further, Patent Document 3 discloses providing a wobble on each layer of a multi-layered record medium and inserting address information into the wobble.
As the feasibility of multi-layered recording increases based on basic research, essential technologies have been identified and invented. Needless to say, various types of servo signals and wobble signals are inevitably necessary in multi-layered media. It is important to manage the characteristics of these signals or define such characteristics as standards in order to improve compatibility and maintain stable quality.
[Patent Document 1] Japanese Patent Application Publication No. 2000-235733
[Patent Document 2] Japanese Patent Application Publication No. 2003-091874
[Patent Document 3] Japanese Patent Application Publication No. 2001-052342
In the case of multi-layered record media, the interference with light reflected from other layers becomes a major problem. If an adjacent record layer is unrecorded, for example, it has a high reflectivity, thereby generating intense reflective light. Since the focus of this reflective light is deviated, the light does not converge on the light receiving element. Nonetheless, this light interferes with the intended light reflected from the record layer being accessed, so that the reflected light does not produce an accurate reading. If the adjacent record layer is prerecorded, on the other hand, its reflectance is low, so that the amount of the interference is small. In the case of a multi-layered structure, while interference occurs with any layers of the multi-layered structure, the interface with the adjacent record layer has the largest detrimental effect.
Various signal characteristics of a record medium can be tuned by use of a large number of parameters such as record film composition, record film thickness, groove depth, and groove width. As shown in Patent Documents 1 and 2, the record characteristics may be mainly tuned by selecting a record film material. Patent Document 1 and 2, however, do not teach the specifications of various signals. The term “various signals” used in the present specification and claims include servo signals such as a tracking error signal and a track cross signal obtained when a light beam crosses the tracks formed as grooves in the medium, a wobble signal that includes information about address and medium rotation speed as formed through the wobbling of a track, etc. In addition to these, of course, the amplitude of a reproduced (RF) signal is included as well. The characteristics of these signals are basically defined in terms of the amplitude thereof, and are normalized by use of a sum signal for the purpose of absorbing differences in the measurement conditions such as the intensity of the light beam, the amplification factor of the circuit, and the like. If light reflected from an adjacent record layer interferes to create an error in the sum signal, however, a failure may occur in that accuracy is lost with respect to the error.
The interference with light reflected from an adjacent record layer significantly varies depending on the optical system. This results in the lack of reliability and reduced compatibility.
When an adjacent record layer is a prerecorded area, its reflectivity is low, and the reflected light is weak. In this case, a failure caused by interference with the record layer being accessed may be small.
If the adjacent record layer is a prerecorded area, the recorded data component of the reflective light, despite its weak intensity, interferes with the light reflected from the record layer being accessed. Since the servo signals have different signal bands, the interference may not cause a problem. Since the wobble signal is close to the recorded data band, however, there is a need to pay a close attention. In the definition of the quality of a wobble signal, it is desirable to define a C/N (carrier-to-noise) ratio in the case of an adjacent record layer being prerecorded.
In this manner, the specification of various signals differs depending on the record status (unrecorded or prerecorded) of the adjacent record layer. Because of this, the method of providing definitions that is designed for the record layer having a one-layer (single-layered) structure cannot achieve stable management of the characteristics, not it can determine standard values.
Accordingly, the object of the present invention is to suppress variation and errors in the measurement of characteristics of various signals caused by the interfering light coming from an adjacent record layer, thereby making it possible to manage, define, and examine media parameters with characteristic values having high reliability, compatibility, and reproducibility.